There are a variety of applications for which thick-film lithography may prove advantageous or even necessary. For instance, thick-film lithography may be used as a plating mold to create metal parts as in the LIGA process. Additionally, there may be applications in thick microelectro mechanical systems or lithographically defined analytical systems such as, for example, chromatography columns or mass spectrometers. Typically, photoresist films greater than 50-100 microns thick are exposed with a synchrotron source; the hard x-rays produced assure good transmission within the photoresist and low diffraction from the mask. In addition, the low run-out from synchrotron sources produces sharp side walls, but synchrotron sources are scarce and expensive. Moreover, long exposure times on the order of hours to days are typical.
Diazonapthoquinone/novolac (DNQ/novolac) resists that are exposed with ultraviolet radiation are used in microcircuit manufacturing. However, these materials suffer from several drawbacks when used in thick (≦50 μm) films. DNQ produces nitrogen gas upon exposure which phase separates prior to diffusing from thick films to create bubbles. Novolac materials form highly absorbing quinones, and the DNQ direct photolysis mechanism typically results in photoresist formulations exhibiting low transmittance. Careful bake steps are required to remove the casting solvent to avoid thermally inducing reactions with the DNQ. In addition, DNQ requires water for proper formation of the soluble, photoproduced acid which leads to requiring long reabsorption times after the initial post-apply bake (PAB). Finally, novolac materials have a tendency to crack, which is particularly problematic for thick films.
A chemically-amplified negative resist, available from MicroChem Corp., Newton, Mass., under the tradename SU-8, circumvents many of these problems. The resist includes monomers and oligomers of bisphenol A, which have been quantitatively protected with glycidyl ether, and a photoacid generator (PAG). UV exposure creates a strong acid which cationically crosslinks the oligomers during a post-exposure bake (PEB) step to form a highly crosslinked network. The resist exhibits high transmission, creates no gas during exposure, and is thermally stable. UV exposures of the resist are typically on the order of minutes, and the cured product provides the best imaging resolution of known resists. However, drawbacks of this resist include solvent development, shrinkage of the cured material, and when used in very thick films, absorption of radiation can result in degraded sidewall profiles. These drawbacks result in poor resolution. The cured resist is also difficult to remove.